(a) Field of the Invention
The present invention relates to a projection lens. More specifically, the present invention relates to a wide-angle projection lens for a projection display.
(b) Description of the Related Art
Recent trends in display device technology have included ultraminiaturization and high precision of unit components as well as the increasing size of displays. Display devices have been making steady progress towards higher quality images on larger screens. Two examples of display devices are the projection display and the flat panel display. Typically, a projection display has a larger set thickness with a lesser image quality relative to a flat panel display. Hence, there is an impetus to improve the technological characteristics of the projection display. To minimize the thickness of the projection TV or monitor, a wide angle of view according to a short projection distance is an essential requirement of the projection lens for projection display.
The projection display is a system that irradiates energy of a defined light distribution formed through a lighting system on a display panel, and enlarges the image of the display panel through a projection lens. In the projection display, the brightness of the projection lens and the central and peripheral illuminance ratio have a direct effect on the brightness and the illuminance of the TV or monitor. Therefore, the projection lens is required to have a low F-number and a high level of peripheral light.
Previous projection lenses developed to satisfy the aforementioned requirements include those disclosed in Japanese Patent Laid-open Publication No. 1993-203871, Japanese Patent Laid-open Publication No. 1995-311338, Japanese Patent Laid-open Publication No. 2003-15033, Japanese Patent Laid-open Publication No. 2003-35870, Japanese Patent Laid-open Publication No. 2003-156683, U.S. Pat. No. 5,973,848, and U.S. Pat. No. 5,644,435.
The projection lens of the Japanese Patent Laid-open Publication No. 1993-203871 merely contributes to simplification of the structure. Japanese Patent Laid-open Publication No. 1995-311338 provides a more enhanced optical performance with a somewhat complex structure. However, the projection lens of Japanese Patent Laid-open Publication No. 1995-311338 does not include any means for satisfying more recent requirements for high resolution or reducing the thickness of the TV.
The projection lenses of Japanese Patent Laid-open Publication No. 2003-15033 and Japanese Patent Laid-open Publication No. 2003-35870 suggest a structure for reducing the thickness of the TV or monitor without maintaining the rotation symmetry of the optical system, and they are disadvantageous in regards to the complex structure with respect to the reduced thickness of the optical system, a steep increase of assembling sensitivity, deterioration of the design performance, and a rise of the production cost. Due to these detrimental aspects, the expected products are evaluated to be of an extremely low usefulness.
Compared with the aforementioned technologies, the projection lens of Japanese Patent Laid-open Publication No. 2003-156683 suggests a structure capable of opticalization, but it is difficult for it to achieve an adequate correction of chromatic aberration as required for projection display, causing color blurring.
The projection lens disclosed in the U.S. Pat. No. 5,973,848 includes a first lens group having a negative refractive power, and second and third lens groups having a positive refractive power, with the first lens group and the third lens group having an aspherical lens. The projection lens of U.S. Pat. No. 5,644,435 includes a first lens group having a negative refractive power, and a second lens group having a negative refractive power. The projection lenses of the both these patents cannot achieve an adequate correction of chromatic aberration.
The size and performance of a projection lens is greatly dependent upon characteristics such as the size of an enlarged image (i.e., panel size), the brightness of the lens, and the zoom factor. In consideration of a panel size of 0.5″ to 1″, the size of the lens on the screen side is generally around 50 mm to 100 mm, causing an escalation in the price. The projection lens is configured as a retrofocus-type optical system so as to secure a long retrofocal length and realize its miniaturization. The retrofocus optical system generally includes a first lens group having a negative refractive power and a second lens group having a positive refractive power. The negative refractive power of the first lens group is extremely high when a wide angle of view and a long retrofocal length are required. A drop in the negative refractive power causes an increase in distortion and chromatic aberration, both of which are difficult to correct. A conventional method for correcting distortion and chromatic aberration involves increasing the number of lenses in the first lens group and arranging a convex lens having a positive refractive power in the first lens group. Another method uses an aspherical lens for the first lens group to correct distortion while minimizing the increment of the number of lenses.
However, these methods involve an increase in the number of lenses and the use of an aspheric surface for the lenses so as to secure a long retrofocal length and a wide angle of view, increasing the size and hence the weight of the optical system and causing a rise of the production cost due to the increased weight and aperture.
The limitation in increasing the number of lenses causes an obstruction to an adequate correction of aberrations. In case of using an aspherical lens, distortion can be easily corrected, but chromatic aberration is hard to correct.
In addition, a ghost or an internal reflection may cause great deterioration of image quality and contrast for a projection TV, and the characteristics of display panels must be taken into consideration in designing the optical system for such a projection TV. Particularly, when the light incident upon the panel from the lighting system is reflected from the panel and enters the projection lens in the reflection panel such as a DMD (Digital Micromirror Display), the light re-reflected from the surface of the projection lens re-enters the panel to cause a deterioration of image quality in the whole or in a specific part of the screen. Therefore, there is a demand for minimizing the reflected light in a design of the projection lens.